1. Field of the Invention
This invention relates to a data information recovering apparatus which recovers the original information data from transmitted data of a (2, 7, 1, 2, 3) Run Length Limited code to which the information data is converted.
2. Related Background Art
Conventionally, when information data in binary form is to be transmitted, it is required to be converted to a train of codes suitable for the transmission channel.
For example, when information data is to be magnetically recorded, the recordable band of data is limited by losses due to the gap in the magnetic head, the space between the magnetic head and the magnetic recording medium, the thickness of the magnetic recording medium, etc. Magnetic recording of information data is performed by magnetic inversion, so that it is very difficult to record information data which contains DC and low frequency components. Especially, when a signal of low frequency components and having only a few magnetic inversions is recorded, the recording system has no well-known self-clocking function so that low reliability results.
From a standpoint of cost, it is desirable to convert the information data to a train of codes suitable for high density recording and record it. In this train of codes, little intersymbol interference or peak shift will occur. However, this train of codes is likely to be influenced by jitter, etc.
A conversion system suitable for magnetic recording and satisfying the above various conditions to some extent and described above may include a well-known (2, 7, 1, 2, 3) Run Length Limited coding technique, which is used in a magnetic disc recording apparatus or the like.
The (2, 7, 1, 2, 3) Run Length Limited coding includes conversion of information data in accordance with a correspondence such as is shown in FIG. 2 or FIG. 5 in which the resulting train of codes contains between adjacent "1s" two to seven successive "0s", and the number of magnetic inversions of whatever train of information data is few during recording, so that it is possible to increase the number of data bits for the density of magnetic inversions.
If a coding system of this type is represented by (a, b, c, d, e) Run Length Limited coding, a represents the smallest number of successive "0"-bits after coding, b represents the largest number of successive "0"-bits after coding, c represents the number of bits before coding, d represents the number of bits after coding and corresponds to c, and e represents the kind of the bit length after coding. Therefore, the (2, 7, 1, 2, 3) Run Length Limited coding expresses that the number of successive "0"-bits after coding is 2 at its minimum and 7 at its maximum, that 2-bit coded data is output for 1-bit input data, and that the kind of bit length after coding is 3. It is to be noted that the (2, 7, 1, 2, 3) Run Length Limited coding technique is disclosed in U.S. Pat. No. 3,689,899 and further description thereof will be omitted.
The resulting information data converted by this (2, 7, 1, 2, 3) Run length Limited coding is recorded together with an index signal for every predetermined quantity of data. When the information data is to be recovered, the index signals are detected and the data following each index signal is reversely converted in accordance with a correspondence table shown in FIG. 2 or in FIG. 5 to recover the information data.
If it is impossible to detect an index signal when the information data is to be recovered, or if a shift occurs among the reproduction data elements between the index signals due to jitter, etc., it will be impossible to recover the correct original data. It is possible to avoid this problem by inserting many index signals during recording, but the density of recorded information data would be lowered.